Method of increasing the water-retentivity of cellulose fibers and the product produced thereby



United States Patent US. Cl. 260-212 Claims ABSTRACT OF THE DISCLOSURE The ability of a carboxy cellulose to retain water is greatly increased by soaking the carboxy cellulose in a salt solution, consisting of a mixture of water-soluble salts, buffering at an initial pH value between about 6.0 and about 7.5, and where the change in pH, i.e., ApH, of the salt solution, after treatment of the carboxy cellulose, is from about 0 to about 0.5 pH units.

The present invention relates to a process for increasing the water-retentitivity of cellulose fibers, and to the products produced by such process. More particularly, it relates to a process for increasing the water-retentivity of carboxy cellulose fibers, and the products produced by such process.

It is known that cellulose-containing materials, such as wood pulp, cotton, cotton linters or rayon have the ability to retain water within and between their fibers. The amount of water retained may equal or exceed the weight of the fibers. However, when large amounts of water are held, the fiber mass is wet to the touch and liquid water can be squeezed out by the application of pressure. Attempts to obtain a water-insoluble, cellulosic material which can absorb large quantities of water and retain such water, even after the application of pressure, have been unsuccessful.

In an attempt to obtain a water-insoluble, cellulosic material which would retain large amounts of water, it has been found that when a carboxy cellulose is soaked in, and reacted with, an aqueous solution comprising a mixture of water-soluble salts buffering at pH values between about 6.0 and about 7.5, and wherein the change in pH, i.e., n-pH, between the initial salt solution and the final salt solution (just prior to filtration of the carboxy cellulose), is from about 0 to about 0.5 pH units, a carboxy cellulose exhibiting markedly improved water-retentive properties is obtained. The percentage increase in retentivity has been shown to be on the order of from about 400% to about 2400%. This greatly increased retentivity, which is achieved quite economically, permits the use of salt-treated carboxy cellulose in a variety of articles where a high degree of water-retentivity is desirable and of prime importance. Such products are baby diapers, paper towels, sanitary napkins, facial tissues, and surgical wadding.

A great variety of cellulosic-containing starting materials can be employed. Exemplary of such cellulosic materials are wood pulps, such as kraft pulp or sulfite pulp,

cotton, cotton linters and rayon. While it is preferred to employ a 6-car-boxy cellulose obtained by oxidizing the cellulosic starting material with nitrogen dioxide, it is to be understood that the present application is not limited thereto. Carboxy celluloses wherein the carboxyl group is substituted at the C or C position can also be employed. It should also be understood that the present invention is not limited to treating the cellulosic starting material with N0 gas as the oxidizing agent. Other oxidizing agents can .be used with equivalent effect. However, as a matter of convenience in describing the invention, the following detailed description of the invention will discuss the treatment of 6-carboxy cellulose, obtained by oxidizing the cellulosic starting material with nitrogen dioxide gas, with the salt mixtures of the present invention.

The cellulosic starting material is reacted with nitrogen dioxide under process conditions well known in the art. It is known that if the cellulosic starting material is fully reacted, a 6-carboxy cellulose having a carboxyl content of about 25% is obtained. This indicates that the 6-hydroxy group on the anhydroglucose monomeric unit in the cellulose chain has been completely converted to 6-carboxyl. Improved water-retentivity is also exhibited at a carboxyl content as low as 3%. However, it is preferred for the purposes of the present invention that the reaction be carried to a point where only from about 10% to about 15% carboxyl content is obtained.

The 6-carboxy cellulose is then immersed or soaked in an aqueous salt solution. The aqueous salt solution consists of a mixture of water-soluble salts, buffering at pH values between about 6.0 and about 7.5. Preferably, the initial pH of the salt solution, which is the pH before the addition of 6-car-boxy cellulose, is 7. It is critical that the change in pH, i.e., ApH, which is the difference between the initial pH of the aqueous salt solution and the pH of the aqueous salt solution containing the suspended 6-carboxy cellulose fibers just prior to filtration, should be as low as possible. The change in pH, which is due to the ion exchange capacity of the carboxy cellulose, should preferably be between about 0 and about 0.5 pH units and even more preferably from about 0 to about 0.1 pH unit.

The concentration of the water-soluble salt mixtures can be from about 1 gram per liter of solution up to the point where the solution becomes saturated. Concentrations of from about 20 grams to about grams per liter of solution are preferred, and, it is especially preferred to employ a concentration of about 40 grams of salt mixture per liter of solution. A concentration of approximately 40 grams per liter of solution, it has been found, results in a fiber which is neither stiff nor harsh;

Any mixture of salts may be used provided their buffering action results in an initial pH for the aqueous salt solution of from about 6.0 to about 7.5. It is also important that the final pH, namely, the pH of the solution just prior to filtration of the salt treated 6-carboxy cellulose fibers, be within the range of from about 0 to about 0.5 pH units of the initial pH of the salt solution alone. Exemplary of the salt solutions containing mixtures of salts conforming to the foregoing requirements, but not limited thereto, are: ammonium bisulfite-ammonium sulfite, ammonium dihydrogen phosphate-diammouium hydrogen phosphate, potassium monohydrogen phosphatepotassium dihydrogen phosphate, potassium bisulfitepotassium sulfite, sodium dihydrogen phosphate-disodium monohydrogen phosphate and sodium bisulfite-sodium sulfite. The preferred salt mixture from the point of view of economy, availability and practicality is sodium bisulfite-sodium sulfite. When employing a sodium bisulfitesodium sulfite mixture it is preferred to employ three parts of sodium sulfite for each part of sodium bisulfite, on a parts by weight basis. Ratios of sodium sulfite to sodium bisulfite of from about 1:1 to about :1 can, however, be employed effectively.

The exact nature of how the mixture of salts interacts with the 6-carboxy cellulose to produce improved waterretentivity is not known. It is thought, however, that in order to obtain maximum retentivity, without subsequent dissolution, the salt 'must be able to react chemically with some, or even most, but not all of the acidic carboxyl groups to form salts. When the above condition is met, a state is reached where maximum swelling of the 6-carboxy cellulose fibers occurs, but dissolution does not occur.

In order to obtain good retentivity, from about 75% or more of the carboxyl groups initially present in a 6- carboxyl cellulose should become salt substituted after being soaked in the salt solution.

The 6-carboxy cellulose can be soaked in the aqueous salt solution for periods of time ranging from less than one hour up to and including 25 hours and more. It is quite interesting to note that 6-carboxy cellulose exhibits marked improvement in its ability to retain water almost instantaneously upon immersion. It has also been found that the longer the soaking or immersion period, the greater the increase in percent water-retentivity exhibited by the 6-carboxy cellulose. Thus, the upper time limit is governed largely by practical economic considerations. The temperature at which the soaking is conducted can be within the range of from about 0 C. to about 100 C. It appears that, with increasing temperature, the ability of the salt soaked 6-carboxy cellulose to retain water increases.

After soaking, the salt treated 6-carboxy cellulose, prepared in accordance with the present invention, is separated from the salt solution by filtration. Preferably, the resulting fibers are then washed and dried, although drying is not an absolute necessity. The resulting fibers can then be used alone or slurried and blended with either synthetic or naturally occurring fibers to produce a highly water-retentive product. It is of interest to note that the salt treated 6-carboxy cellulose fibers retain their high degree of water-retentivity even after they have been washed with water.

The following examples illustrate in detailed fashion the nature of the present invention.

EXAMPLE I 440 grams of flocked pulp (400 g. bone dry weight) was charged into a 22 liter round-bottomed flask. The flask was placed on its side on rollers and rotated at 30 r.p.m. A shallow water bath covered the rollers so that /3 of the exterior surface of the flask was in contact with the water. The water temperature was controlled at 25 C. Nitrogen dioxide gas was passed for 6% hours into the flask through a A" diameter stainless steel tube inserted through the neck of the flask and extending to within about 1 cm. from the bottom. A Teflon cover containing a hole for the stainless steel tube was placed in the neck of the flask. The gas flow was adjusted so an excess of the brown N0 gas slowly escaped through the hole in the cap.

After the 6% hour reaction period, the flask was purged for 10 minutes with a stream of air passed through the stainless steel tube. Then the flask was removed from the rollers, the product was slurried in water, drained on a large Buchner funnel and the product was reslurried in methanol. The methanol slurry was filtered again, the product was washed four times with water and dried overnight in a stream of air at 40 C.

Analysis of the 6-carboxy cellulose, prepared as above, showed a carboxyl content of 12%.

The time of contact with N0 gas may be lengthened to obtain a higher carboxyl level and shortened to obtain a lower carboxyl level.

EXAMPLE II In each of the experiments listed below in Table 1, 2.0 grams of air dried 6-carboxy cellulose fiber, prepared in accordance with Example I, was slurried in 500 mls. of a salt solution. Each of the salt solutions contained a sodium bisulfite-sodium sulfite mixture. The composition of the salt mixture and the percentage water retention are indicated in Table 1. After soaking 18 hours the fiber slurry was stirred using an Osterizer stirrer at maximum speed for forty seconds. The stirring gives a more uniform slurry. The slurry was filtered using a Whatman No. 4 filter paper in a Biichner funnel with an OD. of 67 mm. The fiber pad in the filter was pressed until the fibers and the adhering solution weighed from about 8 to about 11 grams. Weight was determined accurately and the funnel was placed in a vacuum oven without removing the fiber pad. After drying overnight at a temperature of 50 C. and 500 mm. of vacuum, the funnel was placed on a Biichner flask which was connected to a 150 mm. vacuum source. mls. of water containing 0.1% of an anionic surfactant was poured over the surface of the pad. Water was allowed to drain through the pad until the pad surface was free of pools of liquid. Then a glass plate was placed over the funnel and the vacuum was continued for another two minutes. The pad was then removed from the funnel, the filter paper carefully stripped off and the wet pad was weighed accurately. The pad was dried C., atmospheric pressure) and reweighed. The percentage water retention was calculated in accordance with the formula appearing below.

Percent Water Retention (Wet WeightDry Weight) 100 In each of the experiments listed below in Table 2, 2 grams of air dried 6-carboxy cellulose (1.8 g. bone dry weight), prepared in accordance with Example I, and having a carboxyl content of 12.9%, was slurried in 500 mls. of water containing 20 grams of the specific mixture of salts listed below in Table 2. After standing 18 hours, the fibers were filtered using Whatman No. 4 filter paper in a Biichner funnel with an OD. of 67 mm. Suction was used to thoroughly drain the solution. The fibers were slurried in 100 ml. of water and again filtered with suction. After four of these washings to remove the excess salt the fiber pad in the Biichner was treated with 100 mls. of a 0.1% anionic surfactant, Nacconol NR. The subsequent procedures and the calculation of percent water-retentivity are as outlined in Example II above, the only difference being that excess salts were not washed out in Example II.

TABLE 2 Percent Initial Final water Salt mixture pH pH A pH retention K2HPO4+KH2PO4 7. 0 6. 9 0.1 810 7. O 6. 92 0. 08 905 7. 0 6. 95 0. 05 987 6. 5 0. 45 0. 05 1, 030 7. 0 7. 0 0 1, 630 7.02 7. 0 02 2, 400

It is obvious from the foregoing table that when the initial pH is about 7.0 and there is little or no variation between the final pH and the initial pH, i.e., ApH, excellent water-retention is obtained.

EXAMPLE IV In each of the experiments listed below in Table 3, 2 grams of 6-carboxy cellulose, having a carboxyl content of 12%, was soaked in a sodium bisulfite-sodium sulfite solution for varying lengths of time at room temperature. Each of the solutions contained 10 grams of sodium bisulfite and 30 grams of sodium sulfite per liter of solution. The procedure described in Example III was then employed and the percent water-retentivity was then calculated in accordance with the formula of Example II.

The results in Table 3 below indicate that, 1) increased water-retentivity is shown instantaneously upon immersion and (2) the reaction or soaking time can be varied to give increasing water-retentivity.

Table 3 Time (hours) Percent Water retentivity 447 A 451 EXAMPLE V TABLE 4 Percent water retentivity 50. 6 100 do Dissolved EXAMPLE VI A 2 gram sample of 6-carboxy cellulose was soaked in a solution containing 30 grams per liter of sodium sulfite and 10 grams per liter of sodium bisulfite. Time of treatment was 21 hours at room temperature. After the treatment, the liquor was completely drained and the cellulose was washed twice very thoroughly with water. The sample was then weighed and the percent water-retentivity was calculated in accordance with the formula of Example II. The water-retentivity was found to be 580%. This indicates that intermediate drying of the sample immediately after treatment is not essential in order to achieve good retentivity.

EXAMPLE. VII

A sample of 2 grams of 6-carboxy cellulose was treated as per Example VI. It was dried after draining the liquor. The dried sample was mixed with Cellate, which is an unoxidized, untreated kraft pulp produced by Canadian International Paper Co., in diiferent proportions and the mixtures were reslurried in water. Hand sheets were formed from these slurries. The hand sheets were pressed under 8.5 lbs/sq. inch pressure by an hydraulic press and then dried. The percent water retention was calculated in accordance with the formula in Example II.

The results are shown in Table 5. These results show that percent water-retentivity of the blend increases in proportion to the amount of treated 6-carboxy cellulose in the blend. Reslurrying with water did not diminish the effect of the treatment.

EXAMPLE VIII Rayon staple fiber had a water-retentivity of 167% by the test outlined in Example III. After conversion to carboxylated rayon (12.9% carboxyl) and soaking for 18 hours in a solution containing 30 g./l. Na SO and 10 g./l. NaHSO the treated rayon has a water-retentivity of 860%.

What is claimed is:

1. A process for increasing the water-retentivity of 6-carboxy cellulose having a carboxyl content of from about 3% to about 25% by weight which comprises soaking said 6-carboxy cellulose in an aqueous solution comprising a mixture of water-soluble salts bulfering at an initial pH between about 6.0 and about 7.5, said mixture of water-soluble salts being selected from the group consisting of ammonium bisulfite-ammonium sulfite, ammonium dihydrogen phosphate-diammonium hydrogen phosphate, dipotassium monohydrogen phosphate-potassium dihydrogen phosphate, potassium bisulfite-potassium sulfite, sodium dihydrogen phosphate-disodium monohydrogen phosphate, and sodium bisulfite-sodium. sulfite, recovering the salt-treated 6-carboxy cellulose from the aqueous salt solution wherein the final pH is within the range of from about 0 to about 0.5 pH units of the initial pH of the solution.

2. The process as recited in claim 1 wherein the soak ing is conducted at a temperature of from about 0 C. to about C.

3. The process as recited in claim 1 wherein the soaking period is from about less than 1 hour to about 25 hours.

4. The process as recited in claim 1 wherein the conconcentration of the mixture of salts is from about 1 gram per liter of solution up to the saturation point of the solution.

5. The process as recited in claim 1 wherein the salt mixture is sodium bisulfite-sodium sulfite.

6. A cellulosic material having a high degree of waterretentivity which comprises a water-insoluble salt of 6-carboxy cellulose having a carboxyl content of from about 3% to about 25% by weight and having from about 75% or more of said carboxyl groups salt substituted, said 6-carboxy cellulose having been soaked in an equeous solution comprising a mixture of watersoluble salts buffering at an initial pH of from about 6.0 to about 7.5 and wherein the final pH of the aqueous salt solution is within the range of from about '0 to about 0.5 pH units of the initial pH of the solution, said mixture of water-soluble salts being selected from the group consisting of ammonium bisulfite-ammonium sulfite, ammonium dihydrogen phosphate-diammonium hydrogen phosphate, dipotassium monohydrogen phosphate-potassium dihydrogen phosphate, potassium bisulfite-potassium sulfite, sodium dihydrogen phosphatedisodium monohydrogen phosphate and sodium bisulfitesodium sulfite.

7. The cellulosic material as recited in claim 6 wherein the carboxy cellulose was soaked at a temperature of from about 0 C. to about 100 C.

8. The cellulosic material as recited in claim 6 wherein the carboxy cellulose was soaked for a period of from about less than 1 hour to about 25 hours.

7 8 9. The cellulosic material as recited in claim 6 wherein 3,049,433 8/ 1962 Butler 106--197 the concentration of the mixture of salts is from about 3,296,065 1/ 1967 OBrien et al. l62158 1 gram per liter of solution up to the saturation point of OTHER REFERENCES the solution.

10. The cellulosic material as recited in claim 6 Sober et Journal of the American Chemical wherein the salt mixture is sodium bisulfite-sodium 5 y, 76, N0 M 1954, PP- 1711-1712- sulfite.

DONALD E. CZAIA, Primary Examiner References Cited R. W. GRIFFIN, Assistant Examiner UNITED STATES PATENTS 10 2,768,143 10/1956 Henry 106--197 U.S.Cl.X.R. 2,947,645 8/1960 Milne 106-197 260230 

